Carpy-Galy LaTaO${}_4$ undergoes a structural sequence linking the antipolar monoclinic $P{2}_1/c$ ground state and the polar orthorhombic $Cmc{2}_1$ phase through an incommensurately modulated regime. First-principles calculations identify a metastable polar monoclinic $P{2}_1$ phase as a commensurate intermediate in the phase-transition landscape. As a common subgroup of $P{2}_1/c$ and $Cmc{2}_1$, $P{2}_1$ provides a symmetry-connected route between the established polymorphs. Mode-resolved energy surfaces for these symmetry-breaking pathways show that coupling between the respective primary order parameters and an isosymmetric relaxation stabilizes the $P{2}_1$ minimum along both pathways. Consequently, the $P{2}_1/c\to P{2}_1$ transformation has a finite energy barrier, while a restricted soft-mode model retains competing $P{2}_1$ and $Cmc{2}_1$ basins over an illustrative range of harmonic stiffness. Lattice-metric and neutron-diffraction comparisons indicate a shared displacement character between the calculated commensurate state and the experimental IC-o modulation, supporting the use of $P{2}_1$ as a commensurate structural reference for IC-o. Finally, composition-dependent energetics suggest chemical routes for tuning polar-antipolar competition.